Tuesday, February 12, 2013

First, in response to mjs's query about Wi-Fi assisted GPS affecting the iPhone results, I ran the same route with Wi-Fi on (green track) and off (red track):

This is really interesting as it's clear that with the Wi-Fi on, the track "snaps" to the middle of the street, which is where the Google street view car (or similar) would have driven when scanning access points to generate Wi-Fi positioning system data. (This is using my iPhone 3GS, so newer iPhones might behave differently.)

Interesting as this is, the accuracy isn't improved sufficiently with Wi-Fi off to make the iPhone competitive with a dedicated GPS logger.

The second thing I've noticed is the difference in behaviour between my Locosys Genie GT-31 and my Garmin Forerunner 210 when the GPS signal is blocked or poor.

This map shows 10 tracks recorded with the GT-31 along a running track that is suspended below an 8 lane freeway. The concrete roof severely limits the amount of sky that can be seen and the track jumps about and takes a while to get back to normal:

The behaviour of the Garmin is vastly different (the following map shows another 10 tracks):

Garmin has obviously tuned their GPS error correction algorithms specifically for running, and if the GPS data is varying wildly they can assume you're generally going to keep heading in the same direction at the same speed.

The Magellan Switch Up actually performs pretty well in this regard too, I only have 4 logs to map here, but aside from it's "wobble", it doesn't do too badly with a poor signal:

These maps were all produced with my site GPSLog Labs, which has the ability to filter and clean up GPS data, though I'll be needing that feature much less now I'm getting good quality data from the Garmin!